Reaction systems of choice to manufacture acetic acid in high yields, on a large scale with economically viable production rates, include those with a relatively low water (less than 14 wt %) aqueous rhodium catalyst system which includes an iodide salt. See, for example, U.S. Pat. No. 5,144,068 to Smith et al. and U.S. Pat. No. 6,657,078 to Scates et al. So called “low water” processes for making acetic acid have much better carbon monoxide efficiency than conventional Monsanto processes due, in part, to less generation of hydrogen and carbon dioxide by way of the water gas shift reaction.
Commercial systems typically have corrosion metals present in the catalytic medium which result in relatively low levels of iodide salts in the presence of methyl iodide under reaction conditions. In general, conventional wisdom is that corrosion metals (i.e., iron, nickel, chromium, molybdenum, and the like) are not as effective as alkali metals such as lithium in providing inorganic iodide to the system and thereby stabilizing the rhodium catalyst (a significant cost of production) under reduced carbon monoxide pressure as is encountered in a flash vessel. Moreover, corrosion metals have been considered undesirable due to solubility and by-product issues. See U.S. Pat. No. 4,894,477 to Scates et al., Col. 2, line 13 and following, as well as Col. 9, Table 1. As one of skill in the art will be aware, iodide salt containing systems are highly effective as to stabilizing the rhodium from precipitating under reduced carbon monoxide partial pressures as well as maintaining production rates under low water conditions. The rhodium/lithium iodide system has drawbacks, however, notably: (1) the reaction medium is highly corrosive due, in part, to the elevated levels of iodide salt and (2) the rhodium/lithium iodide system tends to generate a plethora of aldehyde-related impurities such as propionic acid, acetaldehyde, crotonaldehyde, higher unsaturated aldehydes, and higher alkyl iodides, all of which are difficult to remove. See Howard et al., Science and Technology in Catalysis 1998, p 64-65 and D. J. Watson, Proceedings of the 17th ORCS Meeting, Marcel Dekker (1998) for more information relating to corrosion and impurities.
Ruthenium and other metals have been considered for their ability to promote higher production rates in combination with iodide salts. Chinese Patent No. 1,562,937 to Haojing Chemical Co., Ltd., discloses use of ruthenium as a co-catalyst at a molar ratio to rhodium of 2.9:1, with a water concentration of 3 to 14.5 wt % and a 15.5 wt % iodide concentration at a rhodium concentration of 1000 ppm (see Table 1). U.S. Pat. No. 5,939,585 to Ditzel et al. disclose use of ruthenium or osmium as a promoter (Claim 1) at a molar ratio to rhodium range of 0.1:1 to 20:1 (Col. 3, lines 58-59) and a water concentration of 0.1 to 7 wt %. U.S. Pat. Nos. 7,368,597 and 7,276,626 both to Gaemers et al. (equivalent to WO 2004/101487 and WO/2004/101488, respectively) show the use of osmium, rhenium, cadmium, mercury, tungsten, ruthenium or zinc as a rate promoter (§0059) at a molar ratio to rhodium of 0.1:1 to 20:1 (§0069) with a water concentration of 0.1 to 30 wt % (§0081). Gaemers et al. also disclose the use of iodide complexes of lanthanide metals, molybdenum, nickel, iron and chromium as stabilizers (§0070). However, Gaemers et al. primarily rely on a ligand to impart catalyst stability.
Other references likewise disclose the use of additional metals in a rhodium/iodide catalyst system for making acetic acid. U.S. Pat. No. 7,053,241 to Torrence discloses the use of tin or ruthenium in a range of molar ratios to rhodium of 0.1:1 to 20:1 (Abstract) at a water concentration of 0.1 to 14 weight % (Col. 4, lines 7-15). The process disclosed in Torrence '241 includes the presence of an iodide ion concentration greater than about 3 wt % as does most of the literature discussing metal promoters/stabilizers in a methanol carbonylation process at water concentrations of less than 14% by weight. United States Publication No. 2008/0071110 to Chen et al., now U.S. Pat. No. 7,671,233, for example, show use of lanthanides, copper, titanium, zirconium, vanadium, manganese, cobalt, palladium, tin, chromium, nickel, molybdenum, or zinc (§0014) as a promoter in a range of molar ratios to rhodium of about 0.1:1 to about 7:1 (110016 and Examples) at a water concentration of 1 to 14 weight %. Chen et al. also discuss the use of yttrium in a molar ratio to rhodium range of 0.09:1 to 5:1 without another stabilizing component; however, in virtually all cases, significant iodide levels are reported and the apparent intended function of the metal promoter/stabilizer is to stabilize inorganic iodide concentration which, in turn, stabilizes the catalyst solution.
Japanese Kokai Patent Application 2005-336105 to Daicel Chemical Industries Ltd., now Japanese Patent No. JP 4657632 B2, discloses a method for manufacturing carboxylic acid in the presence of a rhodium catalyst, lithium iodide at a concentration of 0.1 to 30 wt %, a limited amount of water (15 wt % or less), and at least one element or element-containing compound selected from Zn (in a concentration of 10-5,000 ppm), Sn, Ge, and Pb (in concentrations of 10-20,000 ppm). U.S. Pat. No. 5,218,143 to Jones shows rhodium catalyzed carbonylation with 0.5 to 5 wt % water stabilized with lithium iodide (2-20 wt %; approximately 120:1 to 1200:1 Li:Rh molar ratio) and a Group VI B metal costabilizer, i.e., chromium, molybdenum, or tungsten, in a concentration of 0-10,000 ppm which corresponds to a metal:Rh molar ratio of approximately 0:1 to 276,000:1. The lithium iodide concentrations of Jones are significantly higher than those of the present invention.
Still other metal iodides have been considered as alternative stabilizers to lithium iodide. For instance, U.S. Pat. No. 5,416,237 to Aubigne et al. discloses use of beryllium iodide as a stabilizer, (Col. 3, lines 43-49) using up to 10 weight % water.
Various alternatives to rhodium/lithium iodide systems have been suggested based on laboratory batch unit data, typically including relatively low levels of metal salts, generally at equimolar amounts with rhodium or less. In this regard, see Zhang et al., “Promoting effect of transition metal salts on rhodium catalyzed methanol carbonylation”, Catalysis Communications 7 (2006), pp. 885-888; Ling et al., “Study of the Effects of Rare Earth Metal Additives on Methanol Carbonylation Reaction”, Hua Xue Tong Bao [Notes of Chemistry], Vol. 68, 2005; and Shao et al., “Study of the Effects of Metal Salts on Methanol Carbonylation Reaction”, Journal of Molecular Catalysis (China), Vol. 18, No. 6, December, 2004. So also, it has been suggested to use heteropoly acids of molybdenum and tungsten with rhodium catalysts to make acetic acid, also at relatively low metal concentrations. See Qian et al., “Promoting effect of oxometallic acids, heteropoly acids of Mo, W and their salts on rhodium catalyzed methanol carbonylation”, Catalysis Communications 8 (2007), pp. 483-487. All four documents provide experimental data derived from a batch process, with results determined as soon as 5 minutes into the reaction. These data do not predict results in a continuous process at equilibrium, nor does the data supply information concerning the stability of the catalyst system at reduced carbon monoxide pressure as is seen in a flash vessel of a production unit. With respect to Zhang et al., it is noted that, although metal:rhodium molar ratios (Cr, Fe, Ni, and Zn) of from 2.4 to 4.7 were considered (Table 1), the rate data were determined after 5 minutes. Similarly, with regard to Ling et al., the reaction times were no higher than 55 minutes (Table 1), and only consider a single promoter molar ratio of 1:1 (Nd, Ce, or La:Rh). Furthermore, Shao et al. again provided data after only 10 minutes of reaction time (FIG. 1) for metal:rhodium (Sn, Pb, Cr, and Zr) molar ratios of 0.5:1 to 2.5:1. Note that the tin promoter used was SnCl2. Finally, Qian et al. provided data collected after 5 minutes of reaction time (page 484) for HPA:rhodium molar ratios of from 0.2:1 for phosphotungstic acid (PTA) and sodium phosphotungstate (SPT) to 6:1 for Na2MoO4. In any event, the various papers referred to in this paragraph appear to be directed to identifying metals or metal-containing compositions which provide a substantial iodide concentration to stabilize the rhodium catalyst.
WIPO Publication WO 2006/064178 to BP Chemicals Limited teaches a catalyst system for the production of acetic acid which comprises a rhodium carbonylation catalyst, methyl iodide, and at least one non-hydrohalogenoic acid promoter, such as a heteropoly acid, in the presence or absence of alkali metal iodides, alkaline earth iodides or other components, such as amines or phosphine derivatives, recognized as capable of generating I− by reaction with alkyl iodides such as methyl iodides. The WO '178 publication teaches to optionally include a copromoter capable of generating ionic iodide such as lithium iodide, lanthanide metals, nickel, iron, aluminum, and chromium. It is seen in the Examples which follow that chromium, for example, may be used in accordance with the present invention without forming inorganic iodide at or near theoretically equivalent amounts corresponding to the concentration of chromium added, contrary to the teachings of the WO '178 publication.
As the methanol carbonylation process has been practiced at increasingly lower water concentrations other problems have been found to have arisen. Specifically, operating at this new lower water regime has exacerbated certain impurities in the product acetic acid. As a result, the acetic acid product formed by the above-described low water carbonylation is frequently deficient with respect to the permanganate time owing to the presence therein of small proportions of residual impurities. Since a sufficient permanganate time is an important commercial test which the acid product must meet for many uses, the presence therein of such impurities that decrease permanganate time is objectionable [Ullman's Encyclopedia of Industrial Chemistry, “Acetic Acid”, Volume A1, p. 56, 5th ed]. Of particular concern are certain carbonyl compounds and unsaturated carbonyl compounds, particularly acetaldehyde and its derivatives, crotonaldehyde and 2-ethyl crotonaldehyde (also referred to as unsaturated aldehyde impurities). However, other carbonyl compounds known also to affect the permanganate time are acetone, methyl ethyl ketone, butyraldehyde, and 2-ethyl butyraldehyde. Thus, these carbonyl impurities affect the commercial quality and acceptability of the product acetic acid. If the concentration of carbonyl impurities reaches only 10-15 ppm, the commercial value of the product acetic acid will certainly be negatively affected. As used herein the phrase “carbonyl” is intended to mean compounds which contain aldehyde or ketone functional groups which compounds may or may not possess unsaturation.
It is postulated in an article by Watson, The Cativa™ Process for the Production of Acetic Acid, Chem. Ind. (Dekker) (1998) 75 Catalysis of Organic Reactions, pp. 369-380, that enhanced rhodium catalyzed systems have increased standing levels of rhodium-acyl species which will form free acetaldehyde at a higher rate. The higher rate of acetaldehyde formation can lead to the increased production of permanganate reducing compounds.
The precise chemical pathway within the methanol carbonylation process that leads to the production of crotonaldehyde, 2-ethyl crotonaldehyde and other permanganate reducing compounds is not well understood. One prominent theory for the formation of the crotonaldehyde and 2-ethyl crotonaldehyde impurities in the methanol carbonylation process is that they result from aldol and cross-aldol condensation reactions starting with acetaldehyde. Because theoretically these impurities begin with acetaldehyde, many previously proposed methods of controlling carbonyl impurities have been directed towards removing acetaldehyde and acetaldehyde-derived carbonyl impurities from the reaction system. So also, operation at reduced hydrogen partial pressure and/or reduced methyl iodide has been proposed. See U.S. Pat. No. 6,323,364 to Agrawal, et al., as well as U.S. Pat. No. 6,303,813 to Scates et al., the disclosures of which are incorporated herein by reference.
Conventional techniques used to remove acetaldehyde and carbonyl impurities have included treatment of acetic acid with oxidizers, ozone, water, methanol, amines, and the like. In addition, each of these techniques may or may not be combined with the distillation of the acetic acid. The most typical purification treatment involves a series of distillations of the product acetic acid. Likewise, it is known to remove carbonyl impurities from organic streams by treating the organic streams with an amine compound such as hydroxylamine which reacts with the carbonyl compounds to form oximes followed by distillation to separate the purified organic product from the oxime reaction products. However, this method of treating the product acetic acid adds significant cost to the process.
Despite much effort and substantial need in the art for an improved low water, rhodium catalyzed methanol carbonylation process without elevated levels of inorganic iodide, little progress has been made and the rhodium/lithium iodide system remains the system of choice for commercial production because of the rhodium stability provided under reduced carbon monoxide pressure as is seen in the flasher of a continuous production unit.